1. Field of the Invention
The present invention relates to a polymer electrolyte membrane comprising an inorganic nanoparticle bonded with a proton-conducting group, a solid acid and a proton-conducting polymer, a fuel cell including the same, and a method of preparing the polymer electrolyte membrane.
2. Description of the Related Art
A fuel cell is an electrochemical device, which directly transforms chemical energy of both oxygen and hydrogen contained in a hydrocarbon-based material, such as methanol, ethanol, or natural gas into electrical energy. Since energy conversion processes of fuel cells are very efficient and environmentally friendly, fuel cells have been highlighted and variously developed.
According to the electrolyte that is used, fuel cells can be categorized into phosphoric acid type fuel cells (PAFC), molten carbonate type fuel cells (MCFC), solid oxide type fuel cells (SOFC), polymer electrolyte membrane fuel cells (PEMFC), alkali type fuel cells (AFC), and the like. These fuel cells operate based on the same principle, but have different fuels, different operating temperatures, different catalysts, different electrolytes, etc. Among these fuel cells, PEMFCs are widely considered the most promising fuel cells used in small-sized stationary power generation devices and also transportation systems. This is because PEMFCs have a lower operating temperature, high output density, faster start-up, and a quicker response to a change of output requirement than the other fuel cells mentioned above.
A polymer electrolyte membrane acts as a separator to prevent direct contact between an oxidizing agent and a reducing agent, electrically insulates an anode and a cathode and also acts as a proton conductor. A polymer electrolyte membrane therefore requires good proton conductivity, a good electrical insulating property, low reactant permeability, excellent thermal, chemical and mechanical stability under normal conditions of fuel cell operation, and should be capable of being easily formed in a thin layer and inexpensive.
To satisfy the above requirements, various polymer electrolyte membranes have been developed. For example, a highly fluorinated polysulfonic acid membrane such as NAFION™ membrane is a standard due to excellent durability and performance. However, a NAFION membrane has to be sufficiently humidified to operate well, has to be used at 80° C. or less for preventing moisture loss, and is unstable under operating conditions of a fuel cell because the carbon-carbon bond of a main chain of the NAFION membrane is attacked by oxygen (O2).
In addition, in the case of a direct methanol fuel cell (DMFC), an aqueous methanol solution is supplied to an anode as a fuel. At this time, a part of an unreacted aqueous methanol solution permeates into a polymer electrolyte membrane of the DMFC. The aqueous methanol solution that has permeated into the polymer electrolyte membrane causes a swelling phenomenon in the membrane, thereby being diffused and transferred to a catalyst layer. Such a phenomenon is referred to as “methanol crossover.” Since methanol is directly oxidized in a cathode where electrochemical reduction of hydrogen ions and oxygen occurs, the methanol crossover results in a drop in the electric potential of the cathode, thereby causing a significant decline in the performance of the fuel cell.
Such problems are common problems in fuel cells that use a liquid fuel including polar organic fuels in addition to methanol. Therefore, there is a need for a polymer electrolyte membrane with both reduced methanol crossover and good proton conductivity.